The glutathione S-transferases (GST) are a ubiquitous family of enzymes with dual substrate specificities that perform important biochemical functions including xenobiotic biotransformation and detoxification, drug metabolism, and protection of tissues against peroxidative damage and subsequent inflammatory responses. The basic reaction catalyzed by these enzymes is the conjugation of an electrophilic substrate with reduced glutathione (GSH), which results in either activation or deactivation/detoxification of the substrate. The requirement for conjugating reduced GSH to a wide variety of substrates necessitates a diversity in GST structures in various organisms and cell types.
GSTs are homodimeric or heterodimeric proteins localized in the cell cytosol. The major isozymes share common structural and catalytic properties, and in man have been classified into four major classes, Alpha, Mu, Pi, and Theta. The two largest classes, Alpha and Mu, are identified by their respective protein isoelectric points: pI.about.7.5-9.0 (Alpha) and pI.about.6.6 (Mu). Each GST possesses a common binding site for GSH and a variable hydrophobic binding site. The hydrophobic binding site in each isozyme is specific for particular electrophilic substrates.
In most cases, GSTs perform the beneficial function of deactivating and detoxifying potentially mutagenic and carcinogenic chemicals. However, in some cases their action is detrimental and produces mutagenic and carcinogenic compounds. Some forms of rat and human GSTs are thus reliable preneoplastic markers. Expression of human GSTs in bacterial strains, such as Salmonella typhimurium, used in the well known Ames test for mutagenicity, has helped to establish the role of these enzymes in mutagenesis. Studies have shown that dihalomethanes are more mutagenic in bacterial cells which express human GST than in cells which do not express GST (Thier, R. et al. (1993) Proc. Natl. Acad. Sci. 90: 8576-8580). The mutagenicity of ethylene dibromide and ethylene dichloride is increased in bacterial cells expressing the human Alpha GST, A1-1, while the mutagenicity of aflatoxin B1 is substantially reduced by enhancing the expression of GST (Simula, T. P. et al. (1993) Carcinogenesis 14:1371-6).
GST has been implicated in the acquired resistance of many cancers to drug treatment. Multi-drug resistance occurs when cancer cells are treated with and subsequently become resistant to cytotoxic drugs. Elevated GST levels are observed in some drug resistant cancers. It is believed that the drug being used to treat the cancer is deactivated by the GST-catalyzed GSH conjugation reaction. The increased GST levels protect the cancer cells from all cytotoxic agents for which that GST has affinity. Increased levels of A1-1 in tumors has been linked to drug resistance induced by cyclophosphamide treatment (Dirven H. A. et al. (1994) Cancer Res. 54: 6215-20).
The most abundant mammalian GSTs are the class alpha, mu, and pi enzymes. The biological control of these families is complex and exhibits sex-, age-, tissue-, species-, and tumor-specific patterns of expression. In addition, GSTs are regulated by a structurally diverse range of xenobiotics, and at least 100 chemicals have been identified that induce GST. A significant number of these chemical inducers occur naturally and, as they are found as nonnutrient components in vegetables and citrus fruits, it is apparent that humans are likely to be regularly exposed to such compounds. Class alpha, mu, and pi GST isoenzymes are overexpressed in rat hepatic preneoplastic nodules and the increased levels of these enzymes are believed to contribute to the milti-drug resistant phenotype observed in these lesions. The majority of human tumors and human tumors cell lines express significant amounts of class pi GST. Cell lines selected in-vitro for resistance to anticancer drugs frequently overexpress class pi GST, although overexpression of the class alpha and mu isoenzymes is also often observed.
The discovery of a new human glutathione S-transferase and the polynucleotides encoding it satisfies a need in the art by providing new compositions which are useful in the diagnosis, prevention, and treatment of diseases associated with cell proliferation, in particular, cancers and immune disorders.